Theoretical chemists Lucas de Azevedo Santos, Pascal Vermeeren, F. Matthias Bickelhaupt and Célia Fonseca Guerra of the Vrije Universiteit Amsterdam have rejected a proposal to revise the IUPAC definition of hydrogen bonds. Their work, published in the JACS, shows that the current definition is sufficient.
"In science, definitions are made to convey concepts clearly and accurately," de Azevedo Santos recalled. The International Union of Pure and Applied Chemistry (IUPAC) is the organization responsible for standardizing chemical nomenclature. The hydrogen bond, a crucial interaction in (bio)chemistry, received its official definition from IUPAC in 2011, based on both experimental and theoretical evidence. An important aspect of this definition is the positive charge of the hydrogen atom involved in the hydrogen bond.
A recent article, also published in JACS, argued that the current definition is too narrow, especially in cases where the hydrogen atom in the "hydrogen bond" is negatively charged. However, the theoretical chemistry team refutes this claim and shows that negatively charged hydrogen atoms cannot and do not form hydrogen bonds: they do not accept electrons from the other fragment. Instead, these negatively charged hydrogen atoms donate electrons to the other fragment. The resulting bond therefore falls under another class of already well-defined interactions, such as halogen bonds.
"Our quantum chemical analyses show that negatively charged hydrogen atoms cannot form hydrogen bonds, since their bonding mechanism is completely different," de Azevedo Santos explained. "Hydrogen bonds are formed only when the hydrogen atom can accept electrons, a crucial feature of hydrogen bonds, which is not present in the examples in question. Negatively charged hydrogen atoms cannot do this."
You can read their work, published in the Journal of the American Chemical Society, here: J. Am. Chem. Soc. 2024, online. For more information, contact Prof. Dr. Célia Fonseca Guerra(c.fonsecaguerra@vu.nl).